From: Dave Hansen <dave.hansen@intel.com>
To: Nadav Amit <namit@vmware.com>, Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"x86@kernel.org" <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [RFC PATCH v2 11/12] x86/mm/tlb: Use async and inline messages for flushing
Date: Fri, 31 May 2019 13:13:09 -0700 [thread overview]
Message-ID: <4e0ed5a5-0e5e-3481-e646-3f032f17ac60@intel.com> (raw)
In-Reply-To: <82DB7035-D7BE-4D79-BBC0-B271FB4BF740@vmware.com>
On 5/31/19 12:31 PM, Nadav Amit wrote:
>> On May 31, 2019, at 11:44 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>>
>>
>>> On May 31, 2019, at 3:57 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>>> On Thu, May 30, 2019 at 11:36:44PM -0700, Nadav Amit wrote:
>>>> When we flush userspace mappings, we can defer the TLB flushes, as long
>>>> the following conditions are met:
>>>>
>>>> 1. No tables are freed, since otherwise speculative page walks might
>>>> cause machine-checks.
>>>>
>>>> 2. No one would access userspace before flush takes place. Specifically,
>>>> NMI handlers and kprobes would avoid accessing userspace.
>>>>
>>>> Use the new SMP support to execute remote function calls with inlined
>>>> data for the matter. The function remote TLB flushing function would be
>>>> executed asynchronously and the local CPU would continue execution as
>>>> soon as the IPI was delivered, before the function was actually
>>>> executed. Since tlb_flush_info is copied, there is no risk it would
>>>> change before the TLB flush is actually executed.
>>>>
>>>> Change nmi_uaccess_okay() to check whether a remote TLB flush is
>>>> currently in progress on this CPU by checking whether the asynchronously
>>>> called function is the remote TLB flushing function. The current
>>>> implementation disallows access in such cases, but it is also possible
>>>> to flush the entire TLB in such case and allow access.
>>>
>>> ARGGH, brain hurt. I'm not sure I fully understand this one. How is it
>>> different from today, where the NMI can hit in the middle of the TLB
>>> invalidation?
>>>
>>> Also; since we're not waiting on the IPI, what prevents us from freeing
>>> the user pages before the remote CPU is 'done' with them? Currently the
>>> synchronous IPI is like a sync point where we *know* the remote CPU is
>>> completely done accessing the page.
>>>
>>> Where getting an IPI stops speculation, speculation again restarts
>>> inside the interrupt handler, and until we've passed the INVLPG/MOV CR3,
>>> speculation can happen on that TLB entry, even though we've already
>>> freed and re-used the user-page.
>>>
>>> Also, what happens if the TLB invalidation IPI is stuck behind another
>>> smp_function_call IPI that is doing user-access?
>>>
>>> As said,.. brain hurts.
>>
>> Speculation aside, any code doing dirty tracking needs the flush to happen
>> for real before it reads the dirty bit.
>>
>> How does this patch guarantee that the flush is really done before someone
>> depends on it?
>
> I was always under the impression that the dirty-bit is pass-through - the
> A/D-assist walks the tables and sets the dirty bit upon access. Otherwise,
> what happens when you invalidate the PTE, and have already marked the PTE as
> non-present? Would the CPU set the dirty-bit at this point?
Modulo bugs^Werrata... No. What actually happens is that a
try-to-set-dirty-bit page table walk acts just like a TLB miss. The old
contents of the TLB are discarded and only the in-memory contents matter
for forward progress. If Present=0 when the PTE is reached, you'll get
a normal Present=0 page fault.
> In this regard, I remember this thread of Dave Hansen [1], which also seems
> to me as supporting the notion the dirty-bit is set on write and not on
> INVLPG.
... and that's the erratum I was hoping you wouldn't mention. :)
But, yeah, I don't think it's possible to set the Dirty bit on INVLPG.
The bits are set on establishing TLB entries, not on evicting or
flushing them.
I hope that clears it up.
next prev parent reply other threads:[~2019-05-31 20:13 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-31 6:36 [RFC PATCH v2 00/12] x86: Flush remote TLBs concurrently and async Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 01/12] smp: Remove smp_call_function() and on_each_cpu() return values Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 02/12] smp: Run functions concurrently in smp_call_function_many() Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 03/12] x86/mm/tlb: Refactor common code into flush_tlb_on_cpus() Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 04/12] x86/mm/tlb: Flush remote and local TLBs concurrently Nadav Amit
2019-05-31 11:48 ` Juergen Gross
2019-05-31 19:44 ` Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 05/12] x86/mm/tlb: Optimize local TLB flushes Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 06/12] KVM: x86: Provide paravirtualized flush_tlb_multi() Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 07/12] smp: Do not mark call_function_data as shared Nadav Amit
2019-05-31 10:17 ` Peter Zijlstra
2019-05-31 17:50 ` Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 08/12] x86/tlb: Privatize cpu_tlbstate Nadav Amit
2019-05-31 18:48 ` Andy Lutomirski
2019-05-31 19:42 ` Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 09/12] x86/apic: Use non-atomic operations when possible Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 10/12] smp: Enable data inlining for inter-processor function call Nadav Amit
2019-05-31 6:36 ` [RFC PATCH v2 11/12] x86/mm/tlb: Use async and inline messages for flushing Nadav Amit
2019-05-31 10:57 ` Peter Zijlstra
2019-05-31 18:29 ` Nadav Amit
2019-05-31 19:20 ` Jann Horn
2019-05-31 20:04 ` Nadav Amit
2019-05-31 20:37 ` Jann Horn
2019-05-31 18:44 ` Andy Lutomirski
2019-05-31 19:31 ` Nadav Amit
2019-05-31 20:13 ` Dave Hansen [this message]
2019-05-31 20:37 ` Andy Lutomirski
2019-05-31 20:42 ` Nadav Amit
2019-05-31 21:06 ` Dave Hansen
2019-05-31 21:14 ` Andy Lutomirski
2019-05-31 21:33 ` Nadav Amit
2019-05-31 21:47 ` Andy Lutomirski
2019-05-31 22:07 ` Nadav Amit
2019-06-07 5:28 ` Nadav Amit
2019-06-07 16:42 ` Andy Lutomirski
2019-05-31 6:36 ` [RFC PATCH v2 12/12] x86/mm/tlb: Reverting the removal of flush_tlb_info from stack Nadav Amit
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